Conventional construction machines, such as bulldozers and wheel loaders, are generally provided with a planetary gear type transmission by which forward and rearward movements and speed steps are changed.
FIG. 4 shows an example of a planetary gear type transmission mounted on a construction machine. The planetary gear type transmission 10 is housed in a case 11 and comprises a forward/reverse changeover part 20 and a speed change part 50. In the forward/reverse changeover part 20, a sun gear 22 for reverse movement and a sun gear 24 for forward movement are fixed to an input shaft 13 connected to a drive source 12. A ring gear 26 is arranged around the sun gear 22 for reverse movement and a planetary gear 30 for reverse movement supported by a carrier 28 for reverse movement engages with this ring gear 26 and the sun gear 22 for reverse movement. This carrier 28 for reverse movement can be fixed to the case 11 or opened by a clutch 32 for reverse movement (hereafter referred to as "R clutch"). The ring gear 26 is connected to a carrier 36, which supports a planetary gear 34 for forward movement and two planetary gears of a speed changeover part 50. A ring gear 38 for forward movement, engaged with the planetary gear 34, can be fixed to the case 11 or opened by a clutch 40 for forward movement (hereafter referred to as "F clutch").
In the speed changeover part 50, a first speed sun gear 52, second speed sun gear 54 and third speed sun gear 56 are fixed to an output shaft 14. A second speed planetary gear 58 and a third speed planetary gear 60, which are engaged with the second sun gear 54 and the third speed sun gear 56, are supported by the carrier 36 which supports the planetary gear 34 for forward movement. In addition, the second speed planetary gear 58 and the third speed planetary gear 60 are engaged with a second speed ring gear 62 and a third speed ring gear 64 which are respectively arranged outside these planetary gears. These ring gears 62 and 64 can be fixed to the case 11 or opened by the second speed clutch 66 and the third speed clutch 68.
On the other hand, a carrier 70 connected to the second speed ring gear 62 supports the first speed planetary gear 72. This planetary gear 72 engages with the first speed sun gear 52 and is arranged in ambiance of the sun gear 52 and engaged with a first speed ring gear 76, which is fixed to the case 11 or opened by the first speed clutch 74.
Such planetary gear type transmission 10 guides a hydraulic pressure from a hydraulic circuit 80 to respective clutches 32, 40, 68, 66 and 74 for engagement as shown in FIG. 5 to carry out selection of forward or reverse movement and a speed stage.
A discharging flow passage 82 of a hydraulic pump 81 to be driven by an engine E is connected to a speed valve 84. The speed valve 84 is a four-position changeover valve, for the neutral N and the first, second and third speeds, and is connected to the first speed clutch 74 and an inching valve 85 at the first speed shift position, to the second speed clutch 66 and the inching valve 85 at the second speed shift position, and to the third speed clutch 68 and the inching valve 85 at the third speed shift position. The inching valve 85 is connected to a forward/reverse valve 86 through a flow passage. The forward/reverse valve 86 is a two-position changeover valve and is connected to the F clutch 40 at the forward movement position and to the R clutch 32 at the reverse movement position. The discharging flow passage 82 is branched and the branched flow passage 87 is connected to a modulating valve 88, which is connected by a flow passage to a transmission lubricator 89 and then to an oil reservoir 90.
For selecting the forward first speed, the forward/reverse valve 86 is set at the shift position for forward movement and the speed valve 84 to the first speed shift position by using respective operation levers. A pressure of hydraulic oil from the pump 81 is controlled by the modulating valve 88 and guided to the forward movement clutch 40 and the first speed clutch 74, while being gradually increased, to cause each of the forward movement clutch 40 and the first speed clutch 74 to be engaged. When the forward movement clutch 40 and the first speed clutch 74 are engaged, the forward movement ring gear 38 and the first speed ring gear 76 are fixed to the case 11 and the other clutches are released.
In the event that an engine stall of a construction machine, such as a bulldozer having a transmission as described above, may be caused due to an overload, the operator presses down the inching pedal to move the inching valve 85 in the arrow A direction to reduce the hydraulic pressure guided into the forward movement clutch 40 and to relieve the load on the engine E by slipping the F clutch 40, thus preventing the engine stall.
However, when the operator determines from the sound of the engine E that the engine speed is lowered and the engine stall may be caused by a variation of the vehicle speed, the operator presses down the inching pedal to lower the hydraulic pressure to slip the clutch, and to relieve the load on the engine E by accelerating the engine speed; and, when the revolving speed of the engine E is recovered, the operator stops pressing down of the inching pedal to perform engagement of the clutch. Accordingly, there is a problem in that such operation, wherein the inching pedal should be held down when the engine is overloaded and be released when the engine speed is restored, is often required, is troublesome, and makes the operator physically exhausted.